The present invention relates to a semiconductor device having alignment marks to be used in aligning the semiconductor device by an aligner.
A process for fabricating a semiconductor device includes a lithography step of forming a device pattern on a wafer, as of silicon or others.
In the lithography step, first, a resist is applied by a spin coater or others to a conducting layer or an insulation film laid on a wafer. Then, a mask having dimensions or a layout of a device drawn on is exposed by an aligner, such as a stepper, a scanner or others, in alignment with a prescribed position. Thus, the pattern of the mask is transferred to the resist film. Accuracy of the alignment of the wafer in the transfer by exposing the device pattern is an important element on which production yields of products depend on.
As a method for aligning a wafer in an aligner, FIA (Field Image Alignment), for example is known. An alignment sensor of FIA method comprises a light source for applying illumination to alignment marks formed on a wafer, an image forming optical system for condensing reflected light and diffracted light on the alignment marks to form images of the alignment marks on CCD (Charge Coupled Device) camera, a CCD camera for outputting FIA signals, which is image signals, from the image formed by the image forming system, and a signal processing unit for processing the FIA signals to obtain alignment information of the alignment marks on the wafer.
The conventional standard alignment marks used in the wafer alignment by FIA method will be explained with reference to FIGS. 9A and 9B. FIG. 9A is a top view of the alignment marks, which shows a shape of the alignment marks. FIG. 9B is sectional view of the alignment mark along the line X-X′ in FIG. 9A.
As shown in FIGS. 9A and 9B, alignment marks 104 each of which is, e.g., a rectangular grooves of a 6 μm width and a 70 μm length are formed at a 12 μm pitch side by side in a 250 nm thickness silicon oxide film 102 formed on a silicon wafer 100. An amorphous silicon film 106 is filled in the alignment marks 104. Such alignment marks 104 are formed on a scribe line, which is outside an element region formed on a wafer.
As exemplified in FIG. 9B, a 200 μm thickness silicon oxide film 108 is formed on the upper surface of the above-described structure in a later fabrication step of the semiconductor device. Further on the silicon oxide film 108, BARC (Bottom Anti-Reflection Coating) 110, such as AR5 (Tradename, by Shipley Corporation) or others, is formed in a 95 nm thickness, and a resist film 112 is formed onto the BARC in a 470 nm thickness.
In the alignment of a wafer by FIA optical system, illumination light of a wide-zone wavelength from the light source of the alignment sensor is applied vertically to the alignment marks. Then, reflected light and diffracted light on the alignment marks is captured through the image forming optical system to form the images of the alignment marks on the imaging screen of the CCD camera. FIA signals provided by the CCD camera are processed to sense alignment of the alignment marks on the wafer. Based on thus sensed alignment information, the wafer is aligned.
However, in using the alignment marks shown in FIG. 9A in a fabrication process for a highly integrated semiconductor device of the new era, e.g., 0.13 μm rule DRAM (Dynamic Random Access Memory), the possibility of occurrence of dishing effect in the alignment marks during the CMP (Chemical Mechanical Polishing) process is high. That is, a size of the alignment marks is too large in comparison with a size of the cell pattern, which hinders the upper surface of the region for the alignment marks formed in from being evenly polished, with a result that the region is often unevenly polished into a hollow like a dish.
In the step of forming a metal film by sputtering, the metal film is often unsymmetrically formed on both sides of the edges of the alignment marks.
In a case that a shape of alignment marks is deformed unsymmetrical through the above-described CMP step and the metal film forming step, a central position of the alignment marks cannot be sensed, and a metering error that caused an actual position is erroneously recognized takes place. Such error is called a WIS (Wafer Induced Shift) and is a factor for causing accuracy decrease of alignment of FIA.
A contrast of alignment marks are often changed due to multiple reflection effect of illumination applied by the light source of an alignment sensor, depending on a film structure of a device formed on a wafer, and FIA signals often have a waveform largely changed. Especially, when the edges of alignment marks are sharp, large contrast differences often take place between the edges of the alignment marks and inside the edges. Then, waveforms of FIA signals are changed to have the edges alone of the alignment marks emphasized.
FIG. 9C is a graph of waveforms of FIA signals obtained when the conventional alignment marks 104 shown in FIGS. 9A and 9B are used. As circled in the graph, double edges having the edges alone of the alignment marks 104 emphasized are produced. As a result, the waveforms of the FIA signals become multiplied frequencies having a number of peaks which is twice a number of the alignment marks.
When the waveforms of the FIA signals are changed as shown in FIG. 9C, the FIA signals have different intensities between both edges of the alignment marks, or the waveforms of the FIA signals tend to be deformed. WISs tend to occur.
Central positions of alignment marks cannot be often correctly sensed due to aberrations of the image forming optical system of the alignment sensor. Such errors in sensing central positions of alignment marks are known as TIS (Tool Induced Shift). It is considered that the TIS caused by the alignment sensor itself works with the WIS synergistically to cause large metering errors and further to lower the alignment accuracy.